1. Field of the Invention
The present invention relates to a means of measuring physiological pressures, including blood pressure, intracranial pressure, intrapleural (for evaluation of respiratory function and respiration rate) pressure, pressure within the gastrointestinal system.
2. Description of the Prior Art
Laboratory animals are often used as models of human response to various stimuli and pharmacological agents. Such models are used to project the effect of pharmaceutical compounds, determine the toxicity of chemicals, and to better understand human physiology.
Ony physiological parameter of interest in such evaluations is blood pressure. In many cases, because blood pressure fluctuates with the time of day and is episodic, it is necessary to obtain chronic and frequently sampled measurement of blood pressure within a given animal in order to assess the effects of an agent over a time course. There are several methods which are currently used for chronic measurement of blood pressure. These include the tail cuf method, chronic cannulation, the use of implantable pressure sensors in combination with telemetry, and the use of a vascular access port.
The tail cuff method is well developed with several manufacturers producing such a device. In this case, the animal (usually a rat) is placed in a restraining device. An inflatable cuff is placed on the tail. A blood flow sensor (either plethysmographic or Doppler utrasound) is usually integral to the cuff. The cuff is inflated until blood flow has ceased and is then deflated. The first indication of pulsatile flow and the first indication of continuous flow are noted as the systolic and diastolic pressures, respectively. Some of these devices require that the arteries of the animal be dilated by heating the entire body of the animal to 40.degree. C. or more, causing significant stress on the animal and subsequent artifact. Since these devices require that the animal be restrained, artifact is introduced due to the stress of handling and restraint. In addition, it is not possible to humanely obtain measurements from an animal at frequent intervals with this method, and it is very labor intensive.
Chronic cannulation is the most frequently used method for measurement of blood pressure. With this method, a small catheter is inserted into an artery, typically and carotid, descending abdominal aorta, or femoral artery. The catheter is exteriorized at a point, typically on the back, which generally prevents it from being destroyed by the animal. The catheters from a number of animals may be connected to a single pressure transducer through solenoid valves. A pump is typically used with each animal to continuously backflush the catheter with a heparinized saline or other anticoagulant solution. In addition, a swivel must be used on each catheter to prevent it from becoming tangled as the animal moves about the cage. The solenoid valves and pressure transducers are often connected to a computer to allow frequent sampling of pressure from each animal. This method has several disadvantages: First, since the catheter is long and relatively small in diameter, the higher frequency components of the pressure waveform are lost. Second, even though precautions are taken, the animals often become tangled in the catheter or learn to grab the catheter with their teeth or paws, and subsequently bleed to death. Third, keeping the catheters patent requires considerable maintainance and is thus labor intensive.
Implantable pressure sensors are sometimes used in combination with telemetry. This eliminates some of the disadvantages pointed about above. Konigsburg Instruments manufactures a sensor which is 3.5 mm in diameter. However, this sensor is too large for many applications, and since it is most frequently necessary to mount it in the wall of a vessel, it is subject to fibrous tissue growth over the sensing diaphragm which results in drift of the measured signal. In addition, the nature of the transducer is such that drift is inherent and requires frequent in-vivo calibration.
Miniature solid state sensors mounted on the tip of a catheter have also been used to measure internal body pressures. Some commercially available devices are as small as 1 mm diameter. Because of the inherent instability of these devices they require calibration within a short time prior to use and are suitable only for acute measurements.
In larger animals (dogs, etc.), it is possible to implant a vascular access port in the femoral artery. In this approach, a catheter is attached to a reservoir opposite a diaphragm. The diaphragm can be pierced with a needle connected to a pressure transducer when acquiring pressure measurements and flushing the catheter. The disadvantage of this approach is that it is labor intensive. A sterile protocol is required each time the diaphragm is pierced. In addition, the catheter requires bi-weekly flushing in order to maintain patentcy. If sterile protocol is broken, the animal may develop infection, requiring expensive antibiotics and removal from the study until the infection clears.